Apparatus for polishing thin, flat semiconductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated which oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 20 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, an appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. No. 4,141,180 to Gill, Jr., et al.; U.S. Pat. No. 5,205,082 to Shendon et al; and U.S. Pat. No. 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by a non-uniform pressure applied on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan® head which differs from conventional CMP heads in two major respects is shown. First, the Titan® head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16, are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
The polishing pad 12 is a consumable item used in a semiconductor wafer fabrication process. For instance, under normal wafer fab conditions, the polishing pad must be replaced after a usage of between 12 and 18 hours. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard, incompressible and thus stiffer pads are generally used to achieve planarity. Softer pads are frequently used to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
A problem frequently encountered in using polishing pads in a CMP process for oxide planarization is the rapid deterioration in polishing rates of the oxide with successive wafers. The cause for the deterioration has been shown to be due to an effect known as “pad-glazing” wherein the surface of the polishing pads become smooth such that the pads can no longer hold slurry in-between the fibers. This has been found to be a physical phenomenon on the surface, and is not caused by any chemical reactions between the pad and the slurry.
To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby, restoring the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scrapping the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface., reopen the pores, and thus forms micro scratches in the surface of the pad for improved lifetime of the pad surface. The pad conditioning process can be carried out either during a polishing process, i.e., known as concurrent conditioning, or after a polishing process.
While the pad conditioning process improves pad consistency and its lifetime, conventional apparatus of a conditioning disc is frequently not effective in conditioning a pad surface. For instance, a conventional conditioning disc for use in pad conditioning is shown in FIGS. 2A and 2B. The conditioning disc 30 is formed by embedding or encapsulating diamond particles 32 in a nickel layer 34 coated on the surface 36 of a rigid substrate 38. FIG. 2A is a cross-sectional view of a new conditioning disc with all the diamond particles 32,42 embedded in the nickel layer 34. After repeated usage as a conditioning disc, the cross-sectional view of the disc 30 is shown in FIG. 2B which shows that diamond particle 42 has been lost and the top surfaces of the remaining particles 32 are flattened. The loss of diamond particle from the nickel encapsulation layer 34 occurs frequently when the particle is not deeply embedded in the nickel layer 34. In the fabrication of the diamond particle conditioning disc 30, a nickel encapsulation layer 34 is first mixed with a diamond grit which includes the diamond particles 32,42 and then applied to the rigid substrate 38. The bonding of the diamond particles 32,42 is frequently insecure and thus the particles are easily lost from the nickel layer during usage. The diamond particle 42 which is lost from the nickel encapsulation layer 34 may be trapped between the surfaces of the polishing pad and the wafer and causes severe scratches on the wafer. Another drawback for the diamond conditioning disc is that the pad conditioning efficiency decreases through successive usage of the disc since the top surfaces of the diamond particles are flattened after repeated usage when the diamond grit mechanically abrades the pad surface.
A method for preventing wafer surfaces from being scratched by loose diamond particles that have been dislodged from a conditioned disc is to pre-condition the conditioning disc. Traditionally, this is done in a chemical mechanical polishing apparatus prior to the start of wafer polishing. FIG. 2C shows a conditioning disc 20 is pre-conditioned by polishing disc 12, while the conditioning disc 20 is rotated in a clockwise direction and the polishing pad 12 is rotated in a counter-clockwise direction. The conditioning disc 20 further moves in a linear direction, as shown in FIG. 2C by the ghost lines, in a regular pad conditioning process. In the conventional diamond disc conditioning process, a slurry solution at a flow rate of 200 sccm is utilized while the diamond disc is pressed down onto the polishing disc under a force of 7 lbs. The diamond disc is rotated in a clockwise direction at 63 rpm while the polishing pad is rotated in a counter-clockwise direction at 64 rpm. The pre-conditioning process is carried out for a time period of 30 minutes.
The conventional pre-conditioning process for diamond discs performs adequately in removing loose diamond particles from the disc surface. However, the pre-conditioning process takes at least 30 minutes of valuable fabrication time away from the CMP apparatus and thus reduces the fabrication yield of the machine. It is therefore desirable to conduct a pre-conditioning process on a conditioning disc without occupying a production equipment.
It is therefore an object of the present invention to provide an apparatus for pre-conditioning a conditioning disc that does not have the drawbacks or the shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for carrying out a pre-conditioning process on a conditioning disc in an off-line manner without sacrificing machine time.
It is a further object of the present invention to provide an apparatus for off-line pre-conditioning a conditioning disc that can be carried out without affecting the fabrication yield of the chemical mechanical polishing apparatus.
It is another further object of the present invention to provide an apparatus for off-line pre-conditioning a conditioning disc that is capable of pressing a conditioning disc against a polishing pad under a suitable force.
It is still another object of the present invention to provide an apparatus for off-line pre-conditioning a conditioning disc that is capable of rotating the conditioning disc and a polishing pad in opposite directions.
It is yet another object of the present invention to provide an apparatus for off-line pre-conditioning a conditioning disc that is capable of dispensing a slurry solution in between the surfaces of the conditioning disc and the polishing pad.
It is still another further object of the present invention to provide a method for off-line pre-conditioning a conditioning disc by rotating the conditioning disc against a polishing pad in opposite directions in a pre-conditioning apparatus until all loose particles are removed from the conditioning disc.
It is yet another further object of the present invention to provide off-line pre-conditioning a conditioning disc by suitably adjusting a pressure exerted between the conditioning disc rotated against a polishing pad for a time period of at least 20 minutes until substantially all loose particles are dislodged from the surface of the conditioning disc.